This invention relates to a laser beam generating apparatus, and particularly relates to a light source apparatus for generating high power, high stability, and high efficiency laser beam using solid state laser active medium.
Solid state lasers
A method in which a sold laser active medium is provided in a resonator and the active medium is excited to obtain a laser beam output has been used heretofore. For a laser light source called solid state laser, a semiconductor laser has been used as an excitation light source.
The laser beam source which utilizes a solid state laser active medium is excellent in serviceability. The reason is attributed to the fact that it is easy to adjust the oscillation spectrum of the semiconductor laser to the peak of absorption spectrum of the solid state laser active medium and it is easy to obtain output. The reason is also attributed to the fact that the output power of a solid state laser is proportional to the power of the excitation light source, and then the high power of an all solid state laser can be realized by using the high power semiconductor laser.
End face excitation and side face excitation
The excitation method of solid state laser active media classified roughly into side face excitation and end face excitation depending on the excitation method. The side face excitation is a method in which a medium is excited in the side direction with respect to the direction of laser oscillation, and the end face excitation is a method in which a medium is excited in the same direction as the direction of the laser output.
By applying the end face excitation, high efficient laser beam source is obtained because of spatial overlap between excitation beam and a resonator mode wider than that of the side excitation. Further, single transverse mode is easily obtained because a medium is excited from the same direction as the optical axis of oscillation mode of an oscillator. On the other side, the side face excitation can apply excitation using a plurality of excitation light sources, but the end face excitation can apply only limited number of excitation light sources which are allowed to enter in the end face, and high power is difficult to obtain.
Problems associated with all solid state lasers
It is a problem for obtaining high power of the laser beam source using a solid state laser active medium that the high power involves a problem of heat treatment in addition to the problem of the high power of a semiconductor laser which is an excitation light source. The increased power of an elicitation light source causes the generation of heat in the solid state laser active medium, and the temperature of the active medium increases. To prevent the thermal damage of the active medium, the generated heat should be removed efficiently.
Thermal lensing
In spite of removal of generated heat in the solid state laser active medium, the heat generation inevitably causes thermal gradient between the portion where excitation light impinges into the active medium and the portion which is cooled. Such thermal gradient in an active medium causes the local change in aberration, birefringence, and stress. It has been known that the active medium functions like a thin lens due to such thermal change. This function is called as thermal lensing (literature 1; S. D. Silverstri, P. L. Laporta, and V. Magni, "Pump Power Stability Range of Single-Mode Solid-State Lasers with Rod Thermal Lensing", IEEE J. Quantum Elec., Vol. 23, NO. 11, p 1999-2004 (1987)). The thermal lensing causes the mode change of a resonator, and causes various problems.
The thermal lensing is described briefly herein under. The light reflection effect due to thermal lensing is approximated by the light reflection effect by means of a thin lens. Assuming that the focal length of a thermal lens replaced by approximation is given by f.sub.th, then refracting power d.sub.th is given as described herein under. EQU d.sub.th =1/f.sub.th ( 1)
It is known that d.sub.th is proportional to the absorption power P.sub.pump as shown by the following equation. EQU d.sub.th .varies.P.sub.pump ( 2)
The thermal lensing of a solid state laser active medium due to high excitation causes the condition resembling that a thin lens with varying refracting power is inserted in a resonator. The thermal lensing in the resonator causes the changing in the mode of the resonator. Such unstable condition results in no resonation of the resonator and the output is not obtained.
The design method has been studied for realizing a resonator which resonates in spite of changing of refracting power of a resonator provided with a thermal lens due to thermal lensing (the literature 1 described hereinbefore). The heat removal and thermal lensing are serious problem for obtaining high power, and some device is required to avoid the adverse effect.
Design of conventional laser resonator and problems
Currently, a method has been proposed in which two excitation light sources are used for the end face excitation solid state laser, and one solid state laser active medium is excited by using these light sources (literature 2; A. J. Alfrey, "Simple 1 Micron Ring Laser Oscillators Pumped by Fiber-Coupled Laser Diodes", IEEE J. Quantum Elec., Vol. 30, No. 10, p 2350-p 2355 (1994)).
In this method, a solid state laser active medium 21 provided in a resonator is excited by means of total two excitation laser beam sources 30 and 31 from both sides of the medium 21 as shown in FIG. 1. In the drawing, 11, 12, 13, and 14 are plane mirrors respectively.
However, because two excitation lights L.sub.0 and L.sub.1 is impinged into the same active medium 21 for excitation, thin lenses generated by these excitation lights are formed adjacent closely each other. The total refracting power of these two close thin lenses is given by the sum of refraction power of respective thin lenses. It means that such excitation causes the same thermal lensing as that caused by using one high power excitation light source. Accordingly, such excitation causes the same condition as caused by inserting a medium having a very large refracting power, and the resonator is apt to be unstable.
Assuming that the refracting power due to thermal lensing generated in a solid state laser active medium 21 by means of excitation light sources 30 and 31 is d, and resonator length is L, then the beam matrix for one cycle of the resonator is given by the following equation. ##EQU1## (literature 3; J. M. Eggleston, "Periodic Resonators for Average-Power Scaling of Stable-Resonator Solid-State Lasers" IEEE J. Quantum Elec., Vol. 24, No. 9, p 1821-p 1824, (1988)).
From the condition for self-regeneration of beam of one cycle, the mode radius .omega. is determined as shown herein under. ##EQU2## The condition for the mode radius .omega. to have real value solution is determined as in the following. EQU d&lt;Ld&lt;2 (5)
This corresponds to the stable resonance region of the resonator involving thermal lensing.
FIG. 2 shows the mode radius in the stable resonance region represented in the form of a function of refraction power d. It is obvious that the mode radius is divergent to infinitive on the boundary line in the stable resonance region. This fact is a featured matter for considering the stable condition of a resonator.
The change of output from an excitation light source accompanies inevitably the change of the magnitude of mode of the resonator due to thermal lensing. Thereby, the overlap between the spot radius of the excitation light and the mode radius of the resonator changes. The high extent of overlapping between the spot radius and mode radius results in the low output efficiency and poor quality of the beam.
The use of the flat portion near the limit value in the stable resonance region shown in FIG. 2 results in the high performance laser beam source. However, in the case that only one solid state laser active medium is used as the conventional example described herein above, it can be used only in the range restricted by the equation (5). Therefore it is not possible to expand the stable resonance region no more.
As the result, in the conventional example, the output of 8.5 W is obtained with total excitation of 26 W. The heat of 17.5 W which is not outputted as the output is absorbed in the solid state laser active medium, and the heat should be removed. For this type of resonator design, the output range obtained in the stable resonance region is limited within about 8.5 W.
Problem of Conventional multi-excitation type laser resonator (slab type laser)
The slab type laser is proposed as a light source device provided with increased number of excitation light sources (literature 4: CLEO 1990, CMF4, CMF6). Generally in a slab laser, excitation lights are incident upon a rod like solid state laser active medium along the resonator mode (one type of end face excitation) for excitation in multiple reflection. Thereby the total excitation light source is increased and the high power laser is possible to be obtained.
However, this type of laser is still inconvenient in removal of heat generated in the solid state laser active medium as the above-mentioned conventional example, and further thermal lensing due to excitation generates in adjacent positions, the thermal lensing makes the resonator unstable, this is a problem.
Every conventional examples described hereinbefore do not involve a measure for separating and dispersing the thermal lensing and for suppressing the adverse effect of thermal lensing. The concentration of solid state laser active medium in a resonator not only makes the heat removal problem difficult to be solved but also makes the stable region of the resonator narrow.